It's not massively advertised on the homepage (though in some respects, I think ...

It's not massively advertised on the homepage (though in some respects, I think outside of infosec circles "Firefox Monitor" probably sounds more professional/neutral than "Have I been pwned").

I think they discussed HIBP in the launch announcement: https://blog.mozilla.org/security/2018/11/14/when-does-firef...

It's also in the FAQ: https://support.mozilla.org/en-US/kb/firefox-monitor-faq